Slowly Evolving Elephant Shark Offers Clues about Why Some Fishes Have No Bones

The elephant shark is a relic of a bygone age. Like the coelacanth, it is sometimes referred to as a “living fossil,” a creature alive today that has changed little since it first evolved hundreds of millions of years ago. But besides its prehistoric outward appearance—including the trunklike snout from which it gets its name—the elephant shark boasts a genome that is remarkably different from the genomes of most other living vertebrates, according to a study published online January 8 in Nature. (Scientific American is part of Nature Publishing Group.)

Researchers from the Agency for Science, Technology and Research in Singapore, along with collaborators at several other institutes, sequenced and analyzed the elephant shark’s genome. They hoped to gain insight into the evolution of bony skeletons, because elephant sharks (like other sharks, mantas and rays) have skeletons made of cartilage, a trait that is now rare among vertebrates but was the norm when elephant sharks first evolved.

The scientists discovered that elephant sharks have most of the genes necessary for bone formation, except for a family of genes called the secretory calcium-binding phosphoprotein gene cluster. They think these genes evolved after the elephant shark and its relatives split off from the other fishes that would eventually develop bones.

In their analyses the investigators also found something unexpected: the immune systems of the elephant sharks differ markedly from those of most other vertebrates. Whereas most vertebrates have several different groups of immune helper cells, elephant sharks appear to have only one type.

Because the elephant shark has changed so little from its original form—it is the slowest evolving of all known vertebrates, according to the study—this species offers a rare look into the evolutionary past. It allows researchers to make many more predictions about other ancient species than would be possible from fossils or other preserved remains.